1. Field of the Invention
The present invention relates to a process for producing a low yield ratio and high strength hot rolled steel sheet which has a two-phase structure as hot rolled and shows excellent artificial ageing property after working.
2. Description of the Prior Art
The "two-phase structure" as herein used means a structure which is composed mainly of a ferrite phase, a martensite phase and a small amount of retained austenite phase. The term "low yield ratio" means that the ratio of yield strength/tensile strength as hot rolled and coiled is not higher than 0.6, and the term "high structure" means that the tensile strength is not less than about 40 kg/mm.sup.2. The term "artificial ageing property after working" means the increase of yield strength, which is caused by heating in a temperature range of from about 170.degree. C. to 200.degree. C. after the steel sheet has been placed under a working strain. "An excellent artificial ageing property" indicates that the amount of such increase is large and there is little variation in this property throughout the whole length of the coiled strip sheet.
Recently, in the automobile industry, much effort has been made in reducing the weight of car bodies mainly for the prupose of lowering the fuel consumption rate. Since weight reduction necessitates a thickness reduction of the steel sheet materials, it is essential to use high strength steel sheets.
However, conventionally available high strength steel sheets generally show an excessively high yield ratio so that they exhibit a "spring-back" phenomenon during their press-forming operation. Also, they exhibit poor work-hardening properties during their working so that they are readily susceptible to concentrated local strains and as a result are likely to crack during their deformation working. For all these reasons, the development of wider applications of conventional high strength steel sheets has confronted great difficulties in spite of general recognition of the need for such a product.
Because of this situation, the general tendency among users of steel sheets has been an increasing demand for the development of steel sheets with a yield ratio not higher than about 0.6 and a tensile strength not lower than 40 kg/mm.sup.2, thus satisfying the low yield ratio property (namely a high degree of work-hardening property). Also, it has been desired that these high strength steel sheets exhibit a further increase in the yield strength of the finally formed product through an artificial ageing, such as that caused by passing through a coating-and-drying line (170.degree. C.-200.degree. C.), although such materials possess a fairly high as-formed yield strength because of their high work-hardening property.
A known method for the economical production of a low yield ratio, high strength hot rolled steel sheet, developed by one of the present inventors comprises rapidly cooling a low-carbon steel to a temperature not higher than 350.degree. C. after a finishing hot rolling in the ferrite-austenite two-phase zone (Japanese Patent Application Laid Open No. Sho 51-79628), and a method comprising subjecting a Cr-containing steel to finishing hot rolling in the two-phase zone and coiling at a temperature not higher than 500.degree. C. (Japanese Patent Application No. Sho 53-39163, corresponding to U.S. patent application Ser. No. 22500 of Mar. 21, 1979, incorporated herein by reference). With these methods, it has been possible to economically produce a low yield ratio, high strength, hot rolled steel sheet with less of the spring-back phenomenon during the press forming and possessing a high work-hardening property.
However, the following problems are still to be solved. The steel sheets produced by the above methods do not always possess a satisfactory artificial ageing property after the press forming, and great irregularity of this property is seen throughout the whole length of the coiled strip sheet. For example, when an artificial ageing at 180.degree. C. for 30 minutes is given after a 3% tension deformation, the increase in yield stength is only about 3 to 4 kg/mm.sup.2, or sometimes as low as 1 to 2 kg/mm.sup.2 at local portions of the coil, excluding the work-hardening effect by the tension deformation.